1. Field of the Invention
The invention relates to the acquisition of NMR data from which a chemical shift image may be obtained.
2. Description of the Prior Art
Nuclear magnetic resonance (NMR) techniques have long been used to obtain spectroscopic information about substances. More recently, a number of NMR techniques have emerged for obtaining images of an object. Some techniques now provide spectroscopic information in a spatial image, making it possible, for example, to display an image of a slice of an object in which the concentrations of different chemical substances at each location in the slice are shown. This type of imaging will be referred to herein as "spectroscopic imaging".
A central problem in spectroscopic imaging is the encoding of the spectral information. For example, to perform Fourier imaging, in which the spatial image is ultimately obtained by taking a Fourier transform, the pulse and gradient switching sequence shown in FIG. 1 may be used. During interval 1, a 90.degree. excitation pulse is applied in the presence of gradient G.sub.z applied along the Z axis. This combination excites the atoms in a single slice of an object causing the atoms of that slice to begin to decay from an excited state. As the spins of the atoms decay or evolve, a reversed gradient G.sub.z is applied during interval 2 and phase encoding gradients G.sub.x and G.sub.y are applied during intervals 2 and 3. Gradients G.sub.x and G.sub.y provide an encoding according to the location of the atoms in the selected slice. During interval 4, a 180.degree. refocusing pulse is applied, after which, during interval 5, a spin echo is received and sampled. A data set is built up by applying a number of combinations of the values of G.sub.x and G.sub.y, and the spin echoes of this data set may then be transformed together through a three-dimensional Fourier transform to provide a spectroscopic image of the X-Y plane in which the third dimension shows the high resolution spectrum, as explained below. The spatial resolution can be increased by sampling a larger number of spin echo signals for increasing magnitudes of the phase encoding gradients G.sub.x and G.sub.y.
In the example shown in FIG. 1, the spectral information is encoded directly in the spin echo, and emerges when a Fourier transform of the spin echo is taken. Each chemical compound of a specific element will have one or more characteristic resonances offset in frequency from the basic resonance of that element. Therefore, the frequencies contained in the spin echo will correspond to the compounds of the element being imaged. When a Fourier transform of the spin echo is taken, the resulting spectrum will have a peak corresponding to each compound, the amplitude of the peak reflecting the concentration of that compound. If a data set containing several spin echoes as described above is transformed, an image showing the high resolution spectrum at each location will be produced, showing the concentrations of the compounds at each spatial location.
FIG. 2 illustrates an alternative technique for encoding the spectral information, in which a selective 90.degree. excitation pulse is similarly applied in the presence of gradient G.sub.z during interval 1. Then, during interval 2, gradient G.sub.z is reversed, and during intervals 2 and 3, phase encoding gradient G.sub.y is applied. Also during intervals 2 and 3, an initial prephasing pulse gradient G.sub.x is applied in preparation for applying G.sub.x as an observation gradient during the spin echoes. During interval 4, the 180.degree. refocusing pulse is applied. During interval 5, the observation gradient G.sub.x is again applied for a time period which is centered around the center point of the sampling period of the spin echo. As shown in FIG. 2, several different values of gradient G.sub.y are applied in the Fourier method of imaging to obtain a data set to be transformed.
To encode the spectral information in the sequence of FIG. 2, the timing of the 180.degree. pulse may be changed by an increment dt, as shown in rf sequence (b). This introduces a phase error for all spins which are not resonating at a frequency equal to the detection reference frequency, and the phase error is proportional to the frequency offset, resulting in a phase encoding of the spectral information. W.T. Dixon, in a presentation at the Sixty-ninth Scientific Assembly and Annual Meeting of the Radiological Society of North America, Chicago, Nov. 13-18, 1983, published with revisions as "Simple Proton Spectroscopic Imaging," Radiology, Vol. 153, No. 1 (October, 1984), pp. 189-194, discussed an application of this technique in which protons are imaged and dt is selected so that the water and fat spins, which would be aligned if there were no increment dt, point in opposite directions. Therefore, the sum of the image in which the timing of the 180.degree. pulse is not incremented with the image in which it is incremented by dt will be an image of water alone, but the difference between these images will be an image of fat alone.
The above methods for obtaining spectral information each require a large number of data acquisition sequences or shots, each beginning with an excitation, typically a selective 90.degree. excitation pulse, followed by a phase encoding interval, an echo generating waveform such as a refocusing pulse, and a spin echo sampling interval. The delay between sequences or shots is typically relatively long in relation to the length of each sequence. Therefore, it would be advantageous if more than one spin echo could be sampled for spectral information in each data acquisition sequence.
Techniques used in other areas of NMR imaging gather data from a series of echoes. One such technique is the echo-planar method of imaging, in which a series of echoes are obtained after a single excitation pulse by a series of gradient reversals using a strong field gradient in the presence of a low amplitude orthogonal gradient which remains constant throughout the data acquisition sequence. P. Mansfield, "Spatial Mapping of the Chemical Shift in NMR", J. of Physics D: Applied Physics, Vol. 16 (1983), pp. L235-L238, discusses the application of echo-planar NMR imaging methods to the mapping of chemical shift data spectra. In this technique, as noted above, the echoes result from a series of reversals of a strong gradient field.
It would be advantageous, however, to have a technique for acquiring NMR information including both spatial and spectral information from a series of echoes without the need for repeated reversal of a strong magnetic gradient, which is relatively difficult to achieve in practice. It would also be advantageous to have such a technique which could obtain greater spatial and spectral resolution.